# Homework Help: Force needed to push box down incline plane

1. Sep 16, 2008

### Physics_girl1

1. The problem statement, all variables and given/known data
a) if a force of 86N parallel to the surface of a 20 degree incline plane will push a 120N block up the plane at constant speed, what force parallel to the plane will push it down at constant speed?

2. Relevant equations
$$\Sigma$$F(X)=F(A)-F(fr)-mgcos$$\vartheta$$=0
$$\Sigma$$F(Y)=F(n)-mgsin$$\vartheta$$=o
where F(A)= force applied, F(fr)= friction force, F(n)= normal force, mg=weight, F(X)= sum of forces in x direction, and F(Y)- sum of forces in y direction

3. The attempt at a solution
normal force=41.0424
friction force=26.73

Last edited: Sep 16, 2008
2. Sep 16, 2008

### Physics_girl1

my coefficient of friction for this problem=.652

3. Sep 16, 2008

### Physics_girl1

4. Sep 17, 2008

### ritwik06

Hi
Writing out the equation for the force to push up the block with constant speed, i.e. net force is zero.
120 sin 20(component of weight along incline) +120 cos 20 * u(Force of friction, u is the coefficient)=86
solve for u
u=0.399
Now for the second part:
120 sin 20 +F=120 cos 20 *u
F=120 sin 20-120 cos 20 *u
F=3.9 N
dos this help?????